<!DOCTYPE html>
<html lang="en">
    <head>
        <meta charset="UTF-8" />
        <meta name="viewport" content="width=device-width, initial-scale=1.0" />
        <title>Document</title>
    </head>
    <body>
        <script>
            var detectCycle = function (head) {
                let fast = head
                let slow = head
                while (fast && fast.next) {
                    fast = fast.next.next
                    slow = slow.next
                    if (fast == slow) {
                        fast = head
                        while (fast != slow) {
                            fast = fast.next
                            slow = slow.next
                        }
                        return slow
                    }
                }
                return null
            }
        </script>
    </body>
</html>
